As others have mentioned, printf and sprintf are the functions you want:

$ perl -E 'say sprintf("%.2f", 5.5678)'
5.57
$ perl -E 'printf("%.2f\n", 5.5678)'
5.57
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If you want to round *all* numbers being written to a file then you may want to write a wrapper around that so you don't have to provide the printf format all the time. One way to do that would be by writing a sub-class of Tie::FileHandle::Base or you could write something based on Tie::STDOUT.